Gogeometry Problem 1362


From Gogeometry

Solution

Using :

Solution
  • Define R radius of circle O and r radius of circle Q
  • By construction and symmetry
  • A, O, Q, D are collinear
  • DE=DF, AE=AF
  • AD angle bisector of ∠BAC
  • D symmetric of A by O => ED=EA=FD=FA : AEDF is a diamond
    =>O middle of EF and EF ⊥ AD
  • ΔABC equilateral and AD angle bisector of BAC => ∠CAD=Π/6
  • ∠FAO=Π/6 and ∠AOF=Π/2 => ∠AFO=Π/3
  • ∠AFO intercepts arc FE of circle Q => ∠EDF=Π/3
  • ∠EDF=Π/3 and DE=DF => ΔEDF is equilateral
  • QD=r, OD=R, ΔEDF is equilateral
    => QD=2OD/3 => r=2R/3 and R/r=3/2
  • S=[Circle O]-[Circle Q], S1=[Circle Q]=> S/S1=[Circle O]/[Circle Q]-1
    => S/S1=(R/r)^2-1=9/4-1
    Therefore S/S1=5/4

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